Alkynes: An Introduction to Organic Synthesis

Transcription

1 Alkynes: An Introduction to Organic Synthesis Alkynes Hydrocarbons that contain carbon-carbon triple bonds Acetylene, the simplest alkyne is produced industrially from methane and steam at high temperature Our study of alkynes provides an introduction to organic synthesis, the preparation of organic molecules from simpler organic molecules

2 8.1 Electronic Structure of Alkynes Carbon-carbon triple bond result from sp orbital on each C forming a sigma bond and unhybridized p X and p y orbitals forming a π bond The remaining sp orbitals form bonds to other atoms at 180º to C-C triple bond. The bond is shorter and stronger than single or double Breaking a π bond in acetylene (HCCH) requires 318 kj/mole (in ethylene it is 268 kj/mole) 8.2 Naming Alkynes General hydrocarbon rules apply wuith -yne as a suffix indicating an alkyne Numbering of chain with triple bond is set so that the smallest number possible include the triple bond

3 Diyines, Enynes, and Triynes A compound with two triple bonds is a diyine An enyne has a double bond and triple bond A triyne has three triple bonds Number from chain that ends nearest a double of triple bond double bonds is preferred if both are present in the same relative position Alkynes as substituents are called alkynyl H 3 C CH 2 C C 1-butynyl 8.3 Preparation of Alkynes: Elimination Reactions of Dihalides Treatment of a 1,2 dihaloalkane with KOH or NaOH produces a two-fold elimination of HX Vicinal dihalides are available from addition of bromine or chlorine to an alkene Intermediate is a vinyl halide

4 8.4 Reactions of Alkynes: Addition of HX and X 2 Addition reactions of alkynes are similar to those of alkenes Intermediate alkene reacts further with excess reagent Regiospecificity according to Markovnikov Addition of Bromine and Chlorine Initial addition gives trans intermediate Product with excess reagent is tetrahalide

5 Addition of HX to Alkynes Involves Vinylic Carbocations Addition of H-X to alkyne should produce a vinylic carbocation intermediate Secondary vinyl carbocations form less readily than primary alkyl carbocations Primary vinyl carbocations probably do not form at all Nonethelss, H-Br can add to an alkyne to give a vinyl bromide if the Br is not on a primary carbon 8.5 Hydration of Alkynes Addition of H-OH as in alkenes Mercury (II) catalyzes Markovinikov oriented addition Hydroborationoxidation gives the non-markovnikov product

6 Mercury(II)-Catalyzed Hydration of Alkynes Alkynes do not react with aqueous protic acids Mercuric ion (as the sulfate) is a Lewis acid catalyst that promotes addition of water in Markovnikov orientation The immediate product is a vinylic alcohol, or enol, which spontaneously transforms to a ketone Mechanism of Mercury(II)-Catalyzed Hydration of Alkynes Addition of Hg(II) to alkyne gives a vinylic cation Water adds and loses a proton A proton from aqueous acid replaces Hg(II)

7 Keto-enol Tautomerism Isomeric compounds that can rapidily interconvert by the movement of a proton are called tautomers and the phenomenon is called tautomerism Enols rearrange to the isomeric ketone by the rapid transfer of a proton from the hydroxyl to the alkene carbon The keto form is usually so stable compared to the enol that only the keto form can be observed Hydration of Unsymmetrical Alkynes If the alkyl groups at either end of the C-C triple bond are not the same, both products can form and this is not normally useful If the triple bond is at the first carbon of the chain (then H is what is attached to one side) this is called a terminal alkyne Hydration of a terminal always gives the methyl ketone, which is useful

8 Hydroboration/Oxidation of Alkynes BH 3 (borane) adds to alkynes to give a vinylic borane Oxidation with H 2 O 2 produces an enol that converts to the ketone or aldehyde Process converts alkyne to ketone or aldehyde with orientation opposite to mercuric ion catalyzed hydration Comparison of Hydration of Terminal Alkynes Hydroboration/oxidation converts terminal alkynes to aldehydes because addition of water is non- Markovnikov The product from the mercury(ii) catalyzed hydration converts terminal alkynes to methyl ketones

9 8.6 Reduction of Alkynes Addition of H 2 over a metal catalyst (such as palladium on carbon, Pd/C) converts alkynes to alkanes (complete reduction) The addition of the first equivalent of H 2 produces an alkene, which is more reactive than the alkyne so the alkene is not observed Conversion of Alkynes to cis-alkenes Addition of H 2 using chemically deactivated palladium on calcium carbonate as a catalyst (the Lindlar catalyst) produces a cis alkene The two hydrogens add syn (from the same side of the triple bond)

10 Conversion of Alkynes to trans- Alkenes Anhydrous ammonia (NH 3 ) is a liquid below -33 ºC Alkali metals dissolve in liquid ammonia and function as reducing agents Alkynes are reduced to trans alkenes with sodium or lithium in liquid ammonia The reaction involves a radical anion intermediate (see Figure 8-4) 8.7 Oxidative Cleavage of Alkynes Strong oxidizing reagents (O 3 or KMnO 4 ) cleave internal alkynes, producing two carboxylic acids Terminal alkynes are oxidized to a carboxylic acid and carbon dioxide Neither process is useful in modern synthesis were used to elucidate structures because the products indicate the structure of the alkyne precursor

11 8.8 Alkyne Acidity: Formation of Acetylide Anions Terminal alkynes are weak Brønsted acids (alkenes and alkanes are much less acidic (pk a ~ 25. See Table 8.1 for comparisons)) Reaction of strong anhydrous bases with a terminal acetylene produces an acetylide ion The sp-hydbridization at carbon holds negative charge relatively close to the positive nucleus (see figure 8-5) 8.9 Alkylation of Acetylide Anions Acetylide ions can react as nucleophiles as well as bases (see Figure 8-6 for mechanism) Reaction with a primary alkyl halide produces a hydrocarbon that contains carbons from both partners, providing a general route to larger alkynes

12 Limitations of Alkyation of Acetylide Ions Reactions only are efficient with 1º alkyl bromides and alkyl iodides Acetylide anions can behave as bases as well as nucelophiles Reactions with 2º and 3º alkyl halides gives dehydrohalogenation, converting alkyl halide to alkene 8.10 An Introduction to Organic Synthesis Organic synthesis creates molecules by design Synthesis can produce new molecules that are needed as drugs or materials Syntheses can be designed and tested to improve efficiency and safety for making known molecules Highly advanced synthesis is used to test ideas and methods, answering challenges Chemists who engage in synthesis may see some work as elegant or beautiful when it uses novel ideas or combinations of steps this is very subjective and not part of an introductory course

13 Synthesis as a Tool for Learning Organic Chemistry In order to propose a synthesis you must be familiar with reactions What they begin with What they lead to How they are accomplished What the limitations are A synthesis combines a series of proposed steps to go from a defined set of reactants to a specified product Questions related to synthesis can include partial information about a reaction of series that the student completes Strategies for Synthesis Compare the target and the starting material Consider reactions that efficiently produce the outcome. Look at the product and think of what can lead to it (Read the practice problems in the text) Example Problem: prepare octane from 1-pentyne Strategy: use acetylide coupling

Alkynes An alkyne is a hydrocarbon that contain a Carbon carbon triple bond. Acetylene, the simplest alkyne, widely used in industry for the synthesis of acetaldehyde, acetic acid, vinyl chloride O O H

Chapter 8: Alkynes: an introduction to organic synthesis Alkynes contain a C C triple bond Acetylene: H-C C-H is the common name for ethyne, used as a torch fuel Alkyne nomenclature follows normal hydrocarbon

10.1 Alkynes Alkynes are molecules that incorporate a C C triple bond. 10-1 10.1 Alkynes Given the presence of two pi bonds and their associated electron density, alkynes are similar to alkenes in their

Chapter 11 Alkynes 1 Alkynes Introduction Structure and Bonding Alkynes contain a carbon carbon triple bond. Terminal alkynes have the triple bond at the end of the carbon chain so that a hydrogen atom

Alkynes and Their Reactions Naming Alkynes Alkynes are named in the same general way that alkenes are named. In the IUPAC system, change the ane ending of the parent alkane name to the suffix yne. Choose

DIENES Dienes are alkenes with 2 double bonds. IUPA: Same as alkene, but change -ene to -adiene and use two numbers to locate the two double bonds (number from the end of the chain which makes the smaller

Alkynes Alkynes are compounds that contain a carbon-carbon triple bond Acetylene (common name for Ethyne) overall many reactions are similar to alkene reactions -therefore observe many reactions that are

Alkynes Chapter 7 1 The Carbon is sp hybridized: sp sp 2 There are 2 left over p orbitals for each C: 3 Nomenclature IUPAC: use the infix -yn- to show the presence of a carbon-carbon triple bond. 4 3 2

Andrew Rosen Chapter 7 - Alkenes and Alkynes I 7.1 - Introduction - The simplest member of the alkenes has the common name of ethylene while the simplest member of the alkyne family has the common name

Chapter 10 Problems 10.1-38, 42-43, 45-46. 10. Synthetic Uses of Substitution and Elimination Reactions This chapter discusses how one can use substitution and elimination reactions to synthesize organic

α,β-unsaturated Carbonyl Compounds:* In general, a compound that contains both a carbon-carbon double bond and a carbon-oxygen double bond has properties that are characteristic of both functional groups.

Electrophilic Addition Reactions Electrophilic addition reactions are an important class of reactions that allow the interconversion of C=C and C C into a range of important functional groups. Conceptually,

Nomenclature of Alcohols Systematic names Chapter 11 Alcohols and Ethers 1. Select the longest chain containing the hydroxyl and change the suffix ending of the parent alkane from -e to -ol 2. Number the

Reaction in organic chemistry 1. Definition and classification Chemical reaction is a process in which one or more substances - the reactants are converted to one or more different substances - the products.

Reactions of Alcohols Alcohols are versatile organic compounds since they undergo a wide variety of transformations the majority of which are either oxidation or reduction type reactions. Normally: Oxidation

REACTION AND SYNTHESIS REVIEW A STUDENT WHO HAS MASTERED THIS MATERIAL SHOULD BE ABLE TO PREDICT PRODUCTS, IDENTIFY REACTANTS, GIVE REACTION CONDITIONS, PROPOSE SYNTHESES, AND PROPOSE MECHANISMS (AS LISTED

John E. McMurry www.cengage.com/chemistry/mcmurry Chapter 22 Carbonyl Alpha-Substitution Reactions The α Position The carbon next to the carbonyl group is designated as being in the α position Electrophilic

Unit 2, Lessons 04 and 05: Chemical Reactions of Hydrocarbons The reactivity of hydrocarbons depends on the nature of the C C bonds: single C C bonds, also called sigma bonds (σ), are the least reactive

Alkenes and Alkynes hapter Summary Alkenes have a carbon-carbon double bond and alkynes have a carbon carbon triple bond. Nomenclature rules are given in Sec... Each carbon of a double bond is trigonal,

Oxidation and Reduction of Molecules Oxidation results in an increase in the number of C Z bonds (usually C O bonds) or a decrease in the number of C-H bonds. Reduction results in a decrease in the number

Assistant Lecture Aayad Ammar Alkenes and Alkynes: Structure and Physical Properties Alkenes and alkynes are unsaturated hydrocarbons. The characteristic functional group of an alkene is the carbon-carbon

Carboxylic Acids When a carbonyl carbon also bears a hydroxyl group, then these compounds are appreciably acidic, and are called carboxylic acids. R Carboxylic acids are classified according to the substituent

Alkenes and Alkynes: The Nature of Organic Reactions Alkenes, sometimes called olefins, are hydrocarbons at contain a carbon carbon double bond, C=C, and alkynes are hydrocarbons at contain a carbon carbon

eactions of Alkenes Alkenes generally react in an addition mechanism (addition two new species add to a molecule and none leave) X Y X Y ave already observed using a + electrophile ( or +/ 2 ) that a carbocation

Chapter 10: Organic chemistry fast facts 10.1 Fundamentals of organic chemistry Organic chemistry focuses on the chemistry of compounds containing carbon. A homologous series is a series of compounds with

Alcohols: Reactions The alcohol functional group is -- and its reactions involve cleavage of the - bond or the - bond or In either case, there can be a subsequent substitution, or an elimination to form

LECTURE 6 Electrophilic Addition to Alkynes Alkynes are very similar to alkenes in their behaviour towards electrophiles except that they are slightly less reactive despite the lower steric hindrance and

Carboxylic acid Derivatives Carboxylic acid derivatives are described as compounds that can be converted to carboxylic acids via simple acidic or basic hydrolysis. The most important acid derivatives are

Chapter 8 Homework Alkenes: Reactions and Synthesis SHORT ANSWER Section 8-1 To answer the question(s) below consider the following reaction: When cyclohexene reacts with chlorine in carbon tetrachloride

Chapter 13 Conjugation, Resonance and Dienes What is the conjugation? Conjugation occurs whenever p orbitals are located on three or more adjacent atoms. Conjugated Unsaturated Systems The four p orbitals

CEM 251 Organic Chemistry (Prof. Xuefei Huang) Summary of Important Concepts and Reactions About 60% of the questions in the final will be on materials from chapters 11 to 15. For all the reactions, make

Chapter 12 Alcohols from Carbonyl Compounds: Oxidation-Reduction and Organometallic Compounds Introduction Several functional groups contain the carbonyl group Carbonyl groups can be converted into alcohols

CM 221: rganic Chemistry II Review for Exam 1 Answers Electrophilic Addition Reactions 1. Draw the structure of the carbocation intermediate and the major reaction product for each of the following reactions.

Chapter 8. Alcohols, Phenols, and Ethers Alcohol: A compound that has a hydroxyl group ( ) bonded to a saturated, sp 3 - hybridized carbon atom. Phenol: A compound that has a hydroxyl group bonded to an

Ether, Epoxides and Thiols 2302261 Dr Anawat Ajavakom 1 Ethers and Their Relatives An ether has two organic groups (alkyl, aryl, or vinyl) bonded to the same oxygen atom, R O R. Diethyl ether is used industrially

John E. McMurry http://www.cengage.com/chemistry/mcmurry Chapter 17 Alcohols and Phenols Alcohols and Phenols Alcohols contain an OH group connected to a saturated C (sp 3 ) They are important solvents

IMPTANT CNCEPTS IN ALKYNE CEMISTY SUMMAY F IMPTANT TPICS F ALKYNES AND ALKYNE CEMISTY 1. NMENCLATUE - efer to section 9- of the textbook for IUPAC and common names, and to the chart of functional group

12. Alcohols and Phenols Based on McMurry s Organic Chemistry, 6 th edition Alcohols and Phenols Alcohols contain an OH group connected to a saturated C (sp 3 ) They are important solvents and synthesis

14.1 Introduction to Ethers An ether group includes an oxygen atom that is bonded to TWO R groups: 14.1 Introduction to Ethers Compounds containing ether groups are quite common. R groups can be alkyl,

II Alkanes, Alkenes, and Alkynes Hydrocarbon: Compound composed of only carbon and hydrogen Saturated Hydrocarbons: Compound with only single bonds Unsaturated Hydrocarbons: Compounds with AT LEAST one

Worksheet Addition reactions The presence of electrons in alkenes allows addition reactions to take place. In general: In each case, the bond is broken and the e - are used to form a new bond in the product